1. Field of the Invention
The present invention relates to a power converter equipped with a plurality of thyristor elements connected in series with one another.
2. Description of the Prior Art
A power converter circuit for use in a high-voltage circuitry comprises a plurality of arms each consisting of a stack having a plurality of series-connected thyristor elements. The number of the thyristor elements of each arm is determined on the basis of a normal operating voltage. Other factors concerned with determining the number of the series-connected thyristor elements include an on-off impulse voltage applied sporadically and a lightning impulse voltage, which are also to be taken into consideration. However, since such impulse voltages are limited by an arrester connected in parallel with each arm, it is preferred in view of economy that the number of thyristor elements in each arm be determined in accordance with the limiting voltage of the arrester.
FIG. 1 shows the circuit configuration of one arm in a conventional power converter. In this structure, a series circuit consisting of three thyristor elements 1, 2 and 3 is connected at its one end to an anode terminal 16 via a reactor 18 while being connected at its other end to a cathode terminal 17 via a reactor 19. Gate circuits 13, 14 and 15 are provided to control the thyristor elements 1, 2 and 3, respectively. A snubber circuit 10 consisting of a resistor 4 and a capacitor 7 is connected between the anode and cathode of the thyristor element 1. Similarly, a snubber circuit 11 consisting of a resistor 5 and a capacitor 8 is connected between the anode and cathode of the thyristor element 2, and also another snubber circuit 12 consisting of a resistor 6 and a capacitor 9 between the anode and cathode of the thyristor element 3. An arrester 20 is connected between the terminals 16 and 17. Although it is customary in general to employ a gapless arrester composed of zinc oxide, an ordinary arrester with a gap is also used in some cases.
When none of the thyristor elements is faulty in the circuit configuration of FIG. 1, the voltage applied to each thyristor element is expressed as V.sub.M /N, where V.sub.M is the limit voltage of the arrester and N the total number of the thyristor elements employed. But in case that i (positive integer smaller than N) pieces of the thyristor elements are rendered faulty, the voltage becomes higher to a value expressed by V.sub.M /N-i to eventually increase the possibility of causing damage to the other thyristor elements in normal operation. In order to avoid such a trouble, it is necessary to contrive some adequate means such as provision of a greater number of thyristor elements.
In the arm of another circuit configuration shown in FIG. 2, arresters 21, 22 and 23 are connected in parallel with thyristor elements 1, 2 and 3, respectively. In this structure, the voltage applied to each of the thyristor elements is maintained constant at the limit voltage V.sub.M of each arrester regardless of the number i of faulty thyristor elements, so that there exists no necessity of providing additional thyristor elements. However, in case there occurs a phenomenon that, out of total N pieces of thyristor elements connected in series, i pieces of them fail to conduct properly due to any fault of gate circuits or the like associated therewith, a load current comes to flow in the arresters connected in parallel with such nonconducting thyristor elements. For example, when the second thyristor element 2 alone fails to conduct, a voltage which is equal to the voltage for the entire arm is impressed between the anode and cathode of the second thyristor element 2. However, because of connection of the arrester 22 in parallel with the thyristor element 2 and the characteristic of the power converter to forcibly cause flow of a load current, the terminal voltage of the thyristor element 2 is limited to the terminal voltage V.sub.R of the arrester 22 when the load current I.sub.R determined in accordance with the voltage-current characteristic of the arrester 22 flows therethrough, hence avoiding breakdown of the thyristor element 2. In the meanwhile, the arrester possesses merely a current conducting capability for protection of the thyristor element from any overvoltage of lightning impulse, on-off impulse or the like applied for a short period of time. Therefore, under such a condition that a load current flows repeatedly during a relatively long period of time, thermal breakdown occurs immediately. Furthermore, it is not practical in view of economy to provide a particular arrester capable of withstanding a huge power loss derived from flow of a load current.
The structure of a conventional overvoltage limiter supporting an arrester is shown in FIG. 3, in which a pair of disk-like electrodes 27 and 28 are anchored with screws to both ends of an insulator cylinder 31, and an overvoltage limiting element 24 is housed in a casing constituted of such members. The element 24 is pressed against one electrode 27 by means of a spring 29 disposed between the element 24 itself and the other electrode 28 so that the element 24 is electrically connected, at its surface portion being in contact with the spring 29, to the electrode 28 via a shunt 30. In case the overvoltage limiter is employed in the power converter of FIG. 2, one of the electrodes is connected to the anode of the thyristor element while the other electrode to the cathode thereof.
When there occurs an abnormal state in the power converter of FIG. 2 that the thyristor element 2 alone fails to conduct, a load current flows via the electrodes 28, shunt 30, overvoltage limiting element 24 and electrode 27. And continuous flow of such a load current induces thermal breakdown of the overvoltage limiting element 24 to eventually generate arcing in the insulator cylinder 31, hence increasing the internal gas pressure to break the casing as a result. Consequently, the arc between the two electrodes leaks from the casing and causes damage to peripheral components and may further bring about a fatal accident in the entirety of the power converter.